flatcurry-transform

This package provides a framework to support transformations on FlatCurry programs. A simple transformation on a FlatCurry expression is an operation of type

Expr -> Expr

which can be partially defined and also non-deterministic. For instance, a transformation to remove type annotations can be defined by

unTypeRule :: Expr -> Expr
unTypeRule (Typed e _) = e

Note that this operation is partially defined since it fails on non-"Typed" expressions. However, it is a natural representation of the idea of this transformation.

Similarly, a transformation to remove occurrences of the prelude operation "?" by replacing them by the choice construct of FlatCurry can be defined by

removeQuestionRule :: Expr -> Expr
removeQuestionRule (Comb FuncCall ("Prelude","?") [e1,e2]) = Or e1 e2

The library FlatCurry.Transform.Types defines the type ExprTransformation to support more complex transformations which might require fresh variables or the subterm position. The auxiliary operation makeT lifts a simple transformation into such a general transformation where a name must be provided which is used during debugging. For instance, we can define

unType :: ExprTransformation
unType = makeT "UNTYPE" unTypeRule

removeQuestion :: ExprTransformation
removeQuestion = makeT "REMOVE-?-CAll" removeQuestionRule

The library FlatCurry.Transform.Exec defines the operation transformExpr to apply a transformation as long as possible. If a transformation is non-deterministic, as

(unType ? removeQuestion)

all possible transformations are applied in an unspecified order. The examples (see below) contain code snippets showing the use of the operation transformExpr.

Examples

The subdirectory examples contains various example transformations in the programs

Documentation

More details explaining the ideas and advantages of this kind of program transformations are available in a paper on functional logic program transformations.